A central question in eukaryotic molecular biology is how specific DNA-binding proteins bind regulatory sequences to influence cell function and fate. The steroid/thyroid hormone receptors form a superfamily of ligand-dependent transcription factors that are believed to play a part in such cell function and fate. For example, it is known that these receptors transduce extracellular hormonal signals to target genes that contain specific enhancer sequences (referred to as hormone-response elements, or HREs). Each receptor contains a ligand-binding domain and a DNA-binding domain. The receptor undergoes a conformational change when it binds ligand. This conformational change permits the receptor-ligand complex to bind its cognate response element and thereby regulate transcriptional activity of an associated promoter. Transcriptional activation of promoter drives transcription of an operatively associated structural gene.
Sequence comparison and mutational analyses of hormone receptors, such as the glucocorticoid receptor (GR), have identified functional domains responsible for transcriptional activation and repression, nuclear localization, DNA binding, and hormone binding. The DNA binding domain, which is required in order to activate transcription, consists of 66–68 amino acids of which about 20 sites, including nine cysteines (C1 to C9), are invariant among different receptors. The modular structure of members of this receptor superfamily allows the exchange of one domain for another to create functional, chimeric receptors.
The hormone response elements identified thus far are generally structurally related, but they are in fact functionally distinct. The response elements for GR estrogen receptor (ERE)], and thyroid hormone receptor hormone response elements (TREs)] have been characterized in detail; they each consist of a palindromic pair of ‘half sites’ (Evans, Science 240, 889 (1988); Green and Chambon, Trends in Genetics 4, 309 (1988)]. With optimized pseudo- or consensus response elements, only two nucleotides per half site are different in GRE and ERE [Klock, et al., Nature 329, 734 (1987)]. On the other hand, identical half sites can be seen in ERE and TRE, but their spacing is different [Glass, et al., Cell 54, 313 (1988)]. Moreover, TRE has been shown to mediate transcriptional activation by transfected retinoic acid receptors (RARs) in CV-1 cells whereas non-transfected cells show no response [Umesono et al., Nature 336, 262 (1988)]. In other words, both TR and RAR receptors can activate TREs.
Thus far, however, the response elements for only a few members of the steroid/thyroid superfamily of receptors have been identified. The response elements for many other members of the superfamily, and the relationship between them, if any, remain to be described.